The present invention relates to a data transmission method in a digital CDMA cellular radio network, in which base stations communicate with the mobile station located in their area, in which a mobile station can have a duplex connection with more than one base station simultaneously; and in which signals transmitted from more than one base station can be combined in a mobile station; and in which a signal received from a mobile station in more than one base station can be combined in a base station controller, and in which network the traffic channel transmission is carried out by using a predetermined frame structure, and in which cellular network frames of different types are transmitted between a base station and mobile stations over the radio path.
CDMA is a multiple access method, which is based on the spread spectrum technology and which has been applied recently in cellular radio systems along with the earlier deplayed FDMA and TDMA systems. CDMA has several advantages over the previous methods, such as spectral efficiency and the simplicity of frequency planning.
In a typical mobile telephone environment, the signals between a base station and a mobile station propagate along several paths between the transmitter and the receiver. This multipath propagation is primarily due to the reflection of the signal from the surrounding surfaces. Signals that have propagated along different paths arrive at the receiver at different times due to their different propagation delays. CDMA differs from the conventional FDMA and TDMA in that multipath propagation can be used in receiving a signal. The receiver generally used in the CDMA system is a so-called rake receiver, which consists of one or more rake branches. Each branch is an independent receiver unit, the function of which is to compose and demodulate one received signal component. Each rake branch can be adjusted to synchronize with a signal component propagated along an individual path, and in a conventional CDMA receiver, the signals of the receiver branches are preferably combined, a signal of good quality being thus achieved.
The signal components received by the branches of a CDMA receiver may be transmitted from one or more base stations. In the latter case, it is a question of so-called macrodiversity, i.e. a form of diversity by which the quality of the connection between a mobile station and a base station can be improved. In CDMA cellular radio networks, macrodiversity, which is also referred to by a term "soft handover", is used for ensuring the operation of power control in the fringe areas of base stations and for enabling smooth handovers.
In macrodiversity, a mobile station communicates with two or more base stations simultaneously. The same information is transmitted over all connections. The base stations do not establish independent signalling connections with the mobile station, but all signalling connections have to start from a common switching node. The reliability of the connection between a mobile station and a base station system is based on the combined reliability of several base station connections.
In the known CDMA systems, the combining of signalling messages and user data (speech or data signal) on a traffic channel must be similar over all macrodiversity connections. The traffic between different base stations and a mobile station must thus be identical over all connections. If this were not the case, the mobile station would not be able to combine different signals, and macrodiversity would not be benefited from at all. Therefore, the multiplexing and demultiplexing of signalling and the actual user data must take place in a common location according to the present-day methods.
In the case of a GSM-type network, this is illustrated by FIG. 1. The figure shows three base stations BTS1, BTS2 and BTS3, which are communicating with a mobile station MS. The signal received from the base stations is transmitted along lines 11a-11c to a base station controller BSC, in which the signalling information and the actual data are demultiplexed. The signalling is applied via a signalling unit 10 to the MSC along a separate line 13, and the speech is transmitted via a transcoder unit TRAU further to the MSC along a separate line 14. In this solution, the base stations are merely transmitter/receiver units without signal processing and intelligence. The units provided with intelligence are concentrated within the BTS, wherefore the BSC structure is complicated and difficult to implement. Lines 12a-12c represent signalling connections from the BSC to the base stations BTS1, BTS2, BTS3, the BSC being able to give commands to each BTS along these lines. The signalling transmitted via this path cannot, however, be applied further to the mobile station MS, as in the conventional GSM system, but the signalling intended for a mobile station must be applied via the TRAU unit in order that it can be multiplexed with the speech in a centralized manner. The CDMA-BSC implemented with the known methods would thus differ considerably from the base station controllers of the present-day GSM network.